There is a new era in weed management fast approaching farmers.Monsanto and competitor Dow AgroSciences are on the verge of introducing the next generation of herbicide-resistant corn, soybeans and cotton — genetically modified seeds representing hundreds of millions of dollars in research and the promise of billions in sales.

U.S. regulators have given the green light for Dow’s Enlist corn and soybeans, while final approval is expected shortly for Monsanto’s new Xtend cotton and soybean systems.

There is, however, a potential problem: China.

The marketing of new biotech seeds has long been fraught with perils of one sort or another because of the global nature of grain trading. A seed might be approved in the U.S., but that doesn’t mean other nations are ready for it.

Historically, it’s taken an extra two years to gain approvals in China, which won’t even consider a new engineered seed until it’s been approved by the originating country. But lately, the secretive nation — which happens to be the top importer of soybeans — has become something of a wild card when it comes to unapproved seeds.

“There seems to be a bit of a breakdown in the predictability of that two-year time frame,” said Nathan Fields, director of biotechnology for the National Corn Growers Association in Chesterfield. “We’re just not sure what’s happening in China.”

But what everyone does know is what happens when China enforces its rules.

That was hammered home in November 2013 when China decided, for reasons not totally clear, to test incoming corn shipments.

Regulators found traces of Viptera — a biotech seed introduced in 2010 by Syngenta of Switzerland, but not yet approved by China.

And just like that, U.S. corn’s door to China was effectively slammed shut.

In the months that followed, somewhere around 1 million tons of U.S. corn were rejected, creating havoc in global pricing. Industry estimates suggest the losses to corn growers range from $1 billion to $2.9 billion.